Communication apparatus, communication system, and communication method

ABSTRACT

A communication apparatus including: a transmitter that transmits a first message to another communication apparatus, a receiver that receives a second message from the other communication apparatus, and a processor that performs time synchronization using the first message and the second message, wherein the processor generates information regarding a delay in the communication processing of the first message in the transmitter and the second message in the receiver in a communication layer in communication with the other communication apparatus, and the processor performs the time synchronization with the other communication apparatus based on the information regarding the delay in a time synchronization layer above the communication layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2011-213064, filed on Sep. 28,2011 the entire contents of which are incorporated herein by reference.

FIELD

Embodiments discussed herein are related to a communication apparatus, acommunication system, and a communication method. The communicationapparatus includes a communication apparatus that performs timesynchronization with another communication apparatus.

BACKGROUND

As a protocol for synchronizing distributed clocks on a network, forexample, IEEE 1588 (PTP: Precision Time Protocol) is defined (IEEE Std1588, “IEEE Standard for a Precision Clock Synchronization Protocol forNetworked Measurement and Control Systems”).

In PTP, predetermined messages are exchanged between a device (a“master”) that provides a time reference and a device (a “slave”) thatdepends on a master, and the slave that achieves time synchronizationwith the master using the messages.

For example, a slave calculates the time difference (offset) between amaster and the slave and the route delay (delay) that includes latencyin a network and latency between the devices, corrects the clock basedon the calculation results, thereby synchronizes the slave's time withthe time of the master.

An example of time synchronization via PTP is illustrated in FIG. 1. Asillustrated in FIG. 1, when receiving a synchronization message (Sync)from a master, a slave records the slave time t2 when thesynchronization message is received (see “Timestamps known by slave” inFIG. 1).

In addition, when receiving a supplemental message (Follow_Up) from themaster, the slave extracts, from the supplemental message, a time t1when the synchronization message was transmitted by the master andrecords the time t1. The supplemental message is optional and may not beused. When the supplemental message is not used, the synchronizationmessage includes the time t1 when the synchronization message wastransmitted by the master, and the slave extracts the time t1 from thesynchronization message and records it. The difference tms between thetime t2 and the time t1 equals the sum (offset+delay) of the timedifference (offset) between the master and the slave and the delay(delay).

Then, the slave transmits a delay information request message(Delay_Req) to the master. At this point, the slave records the slavetime t3 when the delay information request message is transmitted.

When receiving the delay information request message from the slave, themaster generates a delay information response message (Delay_Resp) thatincludes a time t4 that is when the delay information request messagewas received and transmits the delay information response message to theslave.

When receiving the delay information response message from the master,which is a response to the delay information request message, the slaveextracts, from the delay information response message, the time t4 whenthe delay information request message was received by the matter andrecords it. The difference tsm between the time t4 and the time t3 isobtained by subtracting the time difference (offset) between the masterand the slave from the delay (delay).

Accordingly, the time difference (offset) between the master and theslave is obtained by the following expression.Offset=[tms−tsm]/2=[(t2−t1)−(t4−t3)]/2  (1)

The slave achieves time synchronization with the master by correctingits time based on the time difference (offset) obtained as describedabove.

Japanese Laid-open Patent Publication No. 2010-213101 discloses atechnique for achieving time synchronization at high precision by, forexample, matching a transmission delay when messages are transmittedfrom a master node to a slave node and a transmission delay whenmessages are transmitted from the slave node to the master node.

Japanese Laid-open Patent Publication No. 2008-193698 discloses atechnique for achieving clock synchronization at high precision in awireless network that provides IP access by, for example, using framepulses as the sampling points of a master counter and slave counter.

Japanese Laid-open Patent Publication No. 2010-190635 discloses atechnique for achieving time synchronization at high precision byavoiding the use of the calculation results of an offset that includesan error for a time correction of the clock unit of the slave.

SUMMARY

According to an aspect of the invention, a communication apparatusincludes: a transmitter that transmits a first message to anothercommunication apparatus, a receiver that receives a second message fromthe other communication apparatus, and a processor that performs timesynchronization using the first message and the second message, whereinthe processor generates information regarding a delay in thecommunication processing of the first message in the transmitter and thesecond message in the receiver in a communication layer in communicationwith the other communication apparatus, and the processor performs thetime synchronization with the other communication apparatus based on theinformation regarding the delay in a time synchronization layer abovethe communication layer.

The object and advantages of the invention is realized and attained bymeans of the elements and combinations particularly pointed out in theclaims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of time synchronization according to PTP.

FIG. 2 illustrates an example of time synchronization performed when amessage is retransmitted.

FIG. 3 illustrates an example of the structure of a communicationsystem.

FIG. 4 illustrates an example of the structure of a master illustratedin FIG. 3.

FIG. 5 is a flowchart illustrating an example of operation of themaster.

FIG. 6 illustrates an example of the structure of a slave illustrated inFIG. 3.

FIG. 7 is a flowchart illustrating an example of operation of the slave.

FIG. 8 illustrates an example of the message sequence of thecommunication system.

FIG. 9 illustrates another example of the structure of the master.

FIG. 10 is a flowchart illustrating an example of operation of themaster illustrated in FIG. 9.

FIG. 11 illustrates another example of the structure of the slave.

FIG. 12 is a flowchart illustrating an example of operation of the slaveillustrated in FIG. 11.

FIG. 13 illustrates another example of the message sequence of thecommunication system.

FIG. 14 illustrates an example of the hardware structure of the master.

FIG. 15 illustrates an example of the hardware structure of the slave.

DESCRIPTION OF EMBODIMENTS

When time synchronization is performed, a delay in communicationprocessing may occur in a layer (a “lower layer”) below the layer (an“upper layer”) in which the time synchronization is performed.

However, since the upper layer does not recognize a delay incommunication processing that occurred in the lower layer, it isdifficult to correct a time error arising from a delay in thecommunication processing that occurs in the lower layer, and timesynchronization may not be performed at high precision.

Embodiments of the present disclosure are described with reference tothe drawings. However, the following embodiments are only examples andthe application of numerous variations and techniques not described inthe following embodiments and modifications are not excluded. That is,it is clear that the embodiments and modifications may be changed andapplied if there is no departure from the spirit of the presentdisclosure.

[1] Example of the Present Embodiment

(1.1) Time Error Arising from a Delay in Communication Processing

An example of time synchronization when a delay due to retransmitting ofa message occurs as a delay in communication processing is illustratedin FIG. 2. Although time synchronization by PTP is illustrated as anexample in FIG. 2, the protocol for time synchronization is not limitedto PTP.

As illustrated in FIG. 2, retransmitting of the synchronization messagefrom, for example, the master to the slave may be performed in acommunication layer (a lower layer) such as the physical layer (PHY) orthe media access control (MAC) layer. This communication layer is alayer below the time synchronization layer (an upper layer) in whichtime synchronization is performed.

In such a case, a delay (ReTxDelay (>0)) occurs, which depends on thenumber of retransmits from when the synchronization message istransmitted by the master to when the synchronization message issuccessfully received by the slave.

In the slave, however, the above delay (ReTxDelay) caused in the lowerlayer is not recognized in the upper layer that performs timesynchronization. Accordingly, the upper layer of the slave performs timesynchronization using the offset obtained through the followingexpression based on the transmitting time t1 of the synchronizationmessage, the receiving time t2 of the synchronization message, thetransmitting time t3 of the delay information request message, and thereceiving time t4 of the delay information response message.Offset=[tms−tsm]/2=[(t2−t1)−(t4−t3)]/2

where the time t1, time t2, time t3, and time t4 meet the followingexpression: 0<t1<t4 and 0<t2<t3.

However, with consideration to the above delay (ReTxDelay), the correctoffset is represented by the following expression (2).Offset=[(tms−ReTxDelay)−tsm]/2=[(t2−ReTxDelay−t1)−(t4−t3)]/2  (2)

Accordingly, the time error caused when the synchronization message isretransmitted once is ReTxDelay/2.

Although FIG. 2 illustrates an example in which the synchronizationmessage is retransmitted once, there may be a case in which thesynchronization message is retransmitted more than once, or a case inwhich another message such as the delay information request message(Delay_Req) is retransmitted once or more than once. In these cases, theabove time error further increases.

There may be a case in which a delay due to adaptive modulation andcoding (AMC) occurs as a delay in communication processing. For example,in a wireless system that uses AMC, the modulation method and code rateduring transmitting of each message may vary depending on the wirelesstransmission environment. In such a case, a delay in communicationprocessing, which causes the above time error, varies depending on thewireless transmission environment.

More specifically, for example, a case in which a modulation method ofquadrature phase shift keying (QPSK) and a code rate of ½ are used iscompared with a case in which a modulation method of 16 quadratureamplitude modulation (16QAM) and a code rate of ½ are used, assumingthat information with the same size is transmitted along the sametransmission path. In this example, the delay in communicationprocessing for the case in which a modulation method of 16QAM is used ishalf the delay in communication processing for the case in which amodulation method of QPSK is used.

Therefore, in the example illustrated in FIG. 2, the lower layer reportsinformation regarding a delay in communication processing of messages tothe upper layer in which time synchronization is performed, and theupper layer performs time synchronization based on the informationreported from the lower layer, so that time synchronization is performedat higher precision.

(1.2) Example of the Structure of a Communication System

An example of the structure of the communication system in this exampleis illustrated in FIG. 3. A communication system 1 in FIG. 3 includes,for example, a master 2 and slaves 3-1, 3-2, and 3-3. When the slaves3-1, 3-2, and 3-3 are not distinguished, they are simply referred tobelow as the slave 3. In addition, the number of slaves 3 is not limitedto the number illustrated in FIG. 3.

The master 2 is a communication apparatus connected to the slave 3either wirelessly or through a wire, and transmits various messages usedfor time synchronization to the slave 3 or receives various messagesused for time synchronization from the slave 3.

In addition, the slave 3 is a communication apparatus connected to themaster 2 either wirelessly or through a wire, and exchanges the variousmessages above with the master 2 to synchronize the time of the slave 3with the time of the master 2.

The master 2 and the slave 3 may transmit or receive data other than thevarious messages above.

(1.3) Example of the Structure of the Master 2

An example of the structure of the master 2 is illustrated in FIG. 4.The master 2 in FIG. 4 includes, for example, a master side clock 21, atime counter 22, a transmitter 26, a receiver 27, and a processing unit28.

The master side clock 21 generates a clock signal. The master side clock21 includes an oscillation circuit having, for example, a quartzoscillator or atomic clock or so on.

The time counter 22 generates and outputs time information by countingthe clock signal generated by the master side clock 21.

The processing unit 28 performs time synchronization using thesynchronization message and delay information response message, which isdescribed later. Accordingly, the processing unit 28 includes, forexample, a time information generating unit 23, a delay informationresponse generating unit 24, and a communication control unit 25.

The time information generating unit 23 generates the synchronizationmessage (Sync), which is an example of time information for reportingthe reference time, using time information output from the time counter22 as reference time. The synchronization message includes thetransmitting time of the synchronization message that is calculatedbased on the reference time. When a supplemental message (Follow_Up) isused to report the transmitting time of the synchronization message, thetransmitting time of the synchronization message is included in thesupplemental message.

The communication control unit 25 transmits, to the transmitter 26, thesynchronization message generated by the time information generatingunit 23 and the delay information response message (Delay_Resp)generated by the delay information response generating unit 24, which isdescribed later, so that they are transmitted to the slave 3.

The communication control unit 25 may also perform retransmit controlwhen the transmitted information is not successfully received by theslave 3. The retransmit control is made based on ACK (acknowledgement)and NACK (negative acknowledgement), which are feedback transmitted fromthe slave 3.

When the delay information request message (Delay_Req) transmitted fromthe slave 3 is successfully received by the receiver 27, ACK istransmitted to the slave 3. On the other hand, when the delayinformation request message from the slave 3 is not successfullyreceived by the receiver 27, NACK may be transmitted to the slave 3.

The communication control unit 25 may calculate the time from when thedelay information request message transmitted from the slave 3 is firstreceived to when the retransmitted delay information request message issuccessfully received based on, for example, an identifier, contained indata transmitted from the slave 3, that indicates whether the data isnew data that was transmitted for the first time or retransmitted data.

The communication control unit 25 may count the number of times thedelay information request message is retransmitted from the slave 3based on the above identifier and may calculate the above delay timebased on the counting result and a processing delay caused for eachretransmit.

In addition, the communication control unit 25 may calculate the delaytime due to communication processing of the delay information requestmessage based on, for example, information such as the modulation methodor code rate of the delay information request message transmitted fromthe slave 3.

This enables the communication control unit 25 to calculate the delaytime caused by communication processing performed in the lower layer.

In the following description, the retransmit status such as the numberof retransmits and the delay time caused by retransmitting and themodulation method and code rate of data may be simply referred to as thereception status.

The communication control unit 25 reports the reception status of thedelay information request message to the delay information responsegenerating unit 24 and instructs it to generate the delay informationresponse message.

If the modulation method and the code rate may be controlled in anadaptive manner in accordance with the propagation path environment inthe communication path between the master 2 and the slave 3, thecommunication control unit 25 may properly set the modulation method andthe code rate of the transmitter 26 and may transmit various types ofinformation depending on the reception quality etc. measured by thereceiver 27.

That is, the communication control unit 25 functions as an example ofthe communication processing unit that controls the communicationprocessing of messages in the transmitter 26 and the receiver 27 in thecommunication layer that is the lower layer, and also outputsinformation regarding a delay in the communication processing.

The transmitter 26 transmits various types of information to the slave 3according to an instruction from the communication control unit 25.

The receiver 27 receives a delay information request message transmittedfrom the slave 3, or receives, from the slave 3, an ACK or NACKindicating whether the slave 3 has successfully received various typesof information transmitted from the master 2.

For example, when the master 2 and the slave 3 are coupled wirelessly,the transmitter 26 and the receiver 27 may be configured as individualor common antennas. In addition, for example, when the master 2 and theslave 3 are coupled through a wire, the transmitter 26 and the receiver27 may be configured as connectors or wired interface devicesappropriate for the transmission path.

The delay information response generating unit 24 obtains, from the timecounter 22, the time when the delay information request message isreceived from the slave 3 as the time information t4, and obtainsinformation (reception status information S1) on the reception status ofthe delay information request message from the communication controlunit 25.

Then, the delay information response generating unit 24 subtracts, fromthe time t4 when the delay information request message is received fromthe slave 3, delays related to retransmitting and communicationprocessing of the modulation method, the code rate or so on using, forexample, the time information t4 and the reception status information S1of the delay information request message. More specifically, the delayinformation response generating unit 24 obtains the corrected time t4′(t4′<t4) by subtracting delays related to communication processing withthe following expression (3) from the time t4, generates a delayinformation response message that includes the corrected time t4′, andtransmits the delay information response message to the communicationcontrol unit 25.t4′=t4−D1  (3)

In the above expression (3), D1 is a delay time that depends on thereception status information S1. For example, when the reception statusinformation S1 is the number of retransmits, the delay informationresponse generating unit 24 may use for D1 in the above correction adelay time (D1 _(rx)), caused by retransmitting, that is calculatedbased on the number of retransmits. When the reception statusinformation S1 is information about the modulation method or code rate,the delay information response generating unit 24 may use for D1 in theabove correction a delay time (D1 _(mcs)), calculated based on themodulation method or code rate, that due to communication processing ofthe delay information request message.

For example, if 100 bytes of information are transmitted in atransmission path with a transmission speed of 100 kbps when themodulation method is QPSK and the code rate is ½, then D1 _(mcs) isrepresented by the following expression (4).D1_(mcs)=100 Bytes*8/100 kbps=8 msec  (4)

If 100 bytes of information is transmitted in a transmission path with atransmission speed of 100 kbps when the modulation method is 16QAM andthe code rate is ½, then D1 _(mcs) is represented by the followingexpression (5).D1_(mcs)=100 Bytes*8/100*2 kbps=4 msec  (5)

That is, the time information generating unit 23 and the delayinformation response generating unit 24 function as an example of thetime synchronization unit that performs time synchronization with theslave 3 based on information regarding delays output from thecommunication control unit 25 in the time synchronization layer that isabove the communication layer.

In the above example, the delay information response generating unit 24corrects the time when the delay information request message wasreceived from the slave 3 from t4 to t4′. However, for example, thereception status information S1 and the receiving time information t4may be transmitted to the slave 3 with the reception status informationS1 and the receiving time information t4 put in the delay informationresponse message, and then the slave 3 may correct the time informationt4 to t4′ based on the reception status information S1.

An example of operation of the master 2 is described in FIG. 5. Asillustrated in FIG. 5, first the receiver 27 receives a delayinformation request message from the slave 3 (step S11).

Next, the delay information response generating unit 24 obtains thereception status information S1 from the communication control unit 25and calculates the delay time D1 based on the obtained reception statusinformation S1 (step S12).

The delay information response generating unit 24 obtains, from the timecounter 22, the receiving time t4 of the delay information requestmessage (step S13) and corrects the receiving time t4 of the delayinformation request message to t4′ (step S14).

Next, the delay information response generating unit 24 generates adelay information response message that includes the corrected time t4′and transmits it to the slave 3 (step S15).

As described above, the master 2 in this example may make a correctionthat subtracts, from the receiving time of the delay information requestmessage transmitted from the slave 3, the delay time due tocommunication processing of the message, so that the delay informationresponse message that includes the corrected time may be transmitted tothe slave 3.

(1.4) Example of the Structure of the Slave 3

An example of the structure of the slave 3 is illustrated in FIG. 6. Theslave 3 in FIG. 6 includes, for example, a slave side clock 31, a timecounter 32, a transmitter 36, a receiver 37, and a processing unit 38.

The slave side clock 31 generates a clock signal. The slave side clock31 includes an oscillation circuit having, for example, a quartzoscillator or atomic clock.

The time counter 32 counts the clock signal generated by the slave sideclock 31.

The receiver 37 receives, from the master 2, a synchronization messagethat is an example of time information, a delay information responsemessage, or an ACK or NACK, which indicate whether the master 2 hassuccessfully received data transmitted from the slave 3.

The transmitter 36 transmits, to the master 2, a delay informationrequest message or an ACK or NACK, which indicate whether datatransmitted from the master 2 has been successfully received, accordingto an instruction by the communication control unit 35.

For example, when the master 2 and the slave 3 are coupled wirelessly,the transmitter 36 and the receiver 37 may be configured as individualor common antennas. For example, when the master 2 and the slave 3 arecoupled through a wire, the transmitter 36 and the receiver 37 may beconfigured as connectors or wired interface devices appropriate for thetransmission path.

The processing unit 38 performs time synchronization using thesynchronization message, the delay information request message, and thedelay information response message. Accordingly, the processing unit 38includes, for example, an information analysis unit 33, a delayinformation request generating unit 34, and the communication controlunit 35.

The communication control unit 35 transmits the delay informationrequest message generated by the delay information request generatingunit 34 to the transmitter 36 so that the message is transmitted to themaster 2.

The communication control unit 35 performs retransmit control if thedelay information request message that is transmitted to the master 2 isnot successfully received by the master 2. This retransmit control isperformed based on an ACK or NACK that the receiver 37 has received fromthe master 3.

When the synchronization message or the delay information responsemessage that is transmitted from the master 2 is successfully receivedby the receiver 37, an ACK may be transmitted to the master 2. On theother hand, when the synchronization message or the delay informationresponse message that is transmitted from the master 2 is notsuccessfully received by the receiver 37, a NACK may be transmitted tothe master 2.

In addition, the communication control unit 35 may calculate, forexample, the time from when the synchronization message transmitted fromthe master 2 is first received to when the retransmitted synchronizationmessage is correctly received based on the identifier that indicateswhether data transmitted from the master 2 is new data that wastransmitted for the first time or retransmitted data, with theidentifier being included in the data.

For example, the communication control unit 35 may count the number oftimes the synchronization message is retransmitted from the master 2based on the above identifier and may calculate the above delay timebased on the counting result and a processing delay generated each timedata is retransmitted.

In addition, the communication control unit 35 may also calculate thedelay time due to the communication processing of the synchronizationmessage based on, for example, information such as on the modulationmethod and the code rate of the synchronization message transmitted fromthe master 2.

As a result, the communication control unit 35 may be able to calculatethe delay time caused by communication processing in a lower layer.

In addition, the communication control unit 35 notifies the informationanalysis unit 33 of the calculated delay time due to communicationprocessing of the synchronization message and the reception status ofthe synchronization message.

If the modulation method and the code rate may be controlled in anadaptive manner depending on the propagation path environment in thecommunication path between the master 2 and the slave 3, thecommunication control unit 35 may properly set the modulation method andthe code rate of the transmitter 36 and may transmit various types ofinformation in accordance with information such as the reception qualitymeasured by the receiver 37.

That is, the communication control unit 35 functions as an example ofthe communication processing unit that controls the communicationprocessing of messages in the transmitter 36 and the receiver 37 in thecommunication layer that is the lower layer, and also outputsinformation regarding a delay in the communication processing.

The delay information request generating unit 34 generates a delayinformation request message according to an instruction by theinformation analysis unit 33 and transfers the generated message to thecommunication control unit 35 so that the message is transmitted to themaster 2. The delay information request generating unit 34 also obtainsthe generation time t3 of the delay information request message from thetime counter 32 and reports the obtained time t3 to the informationanalysis unit 33.

The information analysis unit 33 obtains, from the communication controlunit 35, the time information t1, which is transmitted from the master2, and reception status information S2 about the status when the timeinformation t1 is received, and obtains the time t2 (t2<t3), which iswhen the time information t1 is received from the time counter 32.

When receiving time information from the master 2, the informationanalysis unit 33 instructs the delay information request generating unit34 to transmit the delay information request message to the master 2 andreceives notification of the transmitting time t3 of the delayinformation request message from the time counter 32.

When receiving the delay information response message from the master 2,the information analysis unit 33 obtains, from the delay informationresponse message, the time t4 when the delay information request messagewas received by the master 2.

Then, the information analysis unit 33 subtracts, from the time t2 thatis when the synchronization message is received from the master 2,delays related to retransmitting and communication processing for themodulation method, the code rate or so on using, for example, the timeinformation t2 and the reception status information S2. Morespecifically, for example, the information analysis unit 33 obtains thecorrected time t2′ (t2′<t2) by subtracting the delay related tocommunication processing from time t2 using the following expression(6).t2′=t2−D2  (6)

In the above expression (6), D2 is a delay time that is determined inaccordance with the reception status information S2. For example, whenthe reception status information S2 is the number of retransmits, theinformation analysis unit 33 may use for D2 in the above correction adelay time (D2 _(rx)), caused by retransmitting, that is calculatedbased on the number of retransmits. When the reception statusinformation S2 is information regarding the modulation method or coderate, the information analysis unit 33 may use for D2 in the abovecorrection a delay time (D2 _(mcs)), calculated based on the modulationmethod or code rate, that is due to communication processing of thedelay information request message.

For example, if 100 bytes of information are transmitted in atransmission path with a transmission speed of 100 kbps when themodulation method is QPSK and the code rate is ½, then D2 _(mcs) isrepresented by the following expression (7).D2_(mcs)=100 Bytes*8/100 kbps=8 msec  (7)

If 100 bytes of information are transmitted in a transmission path witha transmission speed of 100 kbps when the modulation method is 16QAM andthe code rate is ½, then D2 _(mcs) is represented by the followingexpression (8).D2_(mcs)=100 Bytes*8/100*2 kbps=4 msec  (8)

When a delay information response message (t4) is received from themaster 2, the information analysis unit 33 calculates the difference(offset) between the time of the master 2 and the time of the slave 3using the following expression (9) and corrects the time counter 32using the calculated offset.Offset={(t2′−t1)−(t4′−t3)}/2={(t2−D2−t1)−(t4−D1−t3)}/2  (9)

The information analysis unit 33 and the delay information requestgenerating unit 34 function as an example of the time synchronizationunit that performs time synchronization with the master 2 based oninformation about delays output from the communication control unit 35in the time synchronization layer that is above the communication layer.

An example of operation of the slave 3 is described in FIG. 7.

As illustrated in FIG. 7, firstly, the receiver 37, the communicationcontrol unit 35, and the information analysis unit 33 obtain the timeinformation t1 and the reception status information S2 (step S21).

Next, the information analysis unit 33 obtains, from the time counter32, the receiving time t2 of the synchronization message (step S22) andcorrects the time t2 to t2′ based on the reception status information S2(step S23).

Then, the information analysis unit 33 and the delay information requestgenerating unit 34 generate a delay information request message andinstruct the communication control unit 35 and the transmitter 36 totransmit the delay information request message (step S24).

The information analysis unit 33 obtains, from the time counter 32, thetransmitting time t3 of the delay information request message (stepS25).

The receiver 37, the communication control unit 35, and the informationanalysis unit 33 receive the delay information response message from themaster 2 and obtain the time t4′ (step S26).

Then, the information analysis unit 33 calculates the offset based onthe time t1, the time t2′, the time t3, and the time t4′ (step S27).

As described above, the slave 3 in this example makes a correction bysubtracting the delay time due to communication processing of thesynchronization message transmitted from the master 2 from the receivingtime of the synchronization message, so that the offset may becalculated based on the corrected time.

Since the slave 3 may perform synchronization with the time of themaster 2 by correcting the time of the slave 3 using the calculatedoffset above, the slave 3 may perform time synchronization at higherprecision.

An example of operation of the above communication system 1 isillustrated in FIG. 8.

As illustrated in FIG. 8, information regarding a communicationprocessing delay of the synchronization message is reported from a lowerlayer of the slave 3 to an upper layer of the slave 3 and a correctionfrom the time t2 to t2′ is made in the upper layer.

Information regarding a delay in communication processing of the delayinformation request message is reported from a lower layer of the master2 to an upper layer of the master 2 and, in this upper layer, acorrection from the time t4 to t4′ is made. Then, the delay informationrequest message that includes the corrected time t4′ is transmitted fromthe master 2 to the slave 3.

In the example described above, the receiving side of messages correctsthe receiving times of the messages based on a delay caused bycommunication processing of the messages, so that the precision of timesynchronization may be improved.

[2] Other Examples of this Embodiment

The receiving side of messages performs a correction in the exampleabove, but the transmitting side of messages may perform the samecorrection.

Examples of the structure and operation of a master 2A, which is amodification of the master 2, and a slave 3A, which is a modification ofthe slave 3A, are described below.

(2.1) Example of the Structure of the Master 2 a

An example of the structure of the master 2A is illustrated in FIG. 9.

The master 2A in FIG. 9 includes, for example, the master side clock 21,the time counter 22, the transmitter 26, the receiver 27, and aprocessing unit 28A.

The master side clock 21 generates a clock signal. The master side clock21 includes, for example, an oscillation circuit having, for example, aquartz oscillator or atomic clock.

The time counter 22 counts the clock signal generated by the master sideclock 21.

The processing unit 28A performs time synchronization using thesynchronization message, the supplemental message, and the delayinformation response message. Accordingly, the processing unit 28Aincludes, for example, a synchronization information generating unit 30,a supplemental information generating unit 29, the delay informationresponse generating unit 24, and the communication control unit 25.

The synchronization information generating unit 30 generates asynchronization message, which is an example of time information, basedon the reference time obtained from the counting result by the timecounter 22.

In addition, the supplemental information generating unit 29 obtains thetransmitting time of the synchronization message generated by thesynchronization information generating unit 30 from the time counter 22and generates a supplemental message that includes the obtainedtransmitting time. When using a supplemental message, thesynchronization message may be a message that doesn't include thetransmitting time of the synchronization message.

The communication control unit 25 transmits, to the transmitter 26, thesynchronization message generated by the synchronization informationgenerating unit 30, the supplemental message generated by thesupplemental information generating unit 29, and the delay informationresponse message generated by the delay information response generatingunit 24, so that they are transmitted to the slave 3A.

The communication control unit 25 may also perform retransmit controlwhen the transmitted information is not successfully received by theslave 3A. The retransmit control is made based on an ACK or NACK, whichare feedback transmitted from the slave 3A.

When the delay information request message transmitted from the slave 3Ais successfully received by the receiver 27, an ACK is transmitted tothe slave 3A. On the other hand, when the delay information requestmessage from the slave 3A is not successfully received by the receiver27, a NACK may be transmitted to the slave 3A.

The communication control unit 25 may calculate the time from when thesynchronization message is first transmitted to when the synchronizationmessage that is successfully received by the slave 3A is transmitted.

Alternatively, the communication control unit 25 may count the number oftimes the synchronization message is retransmitted and the number oftimes a NACK is received from the slave 3A and may calculate the abovedelay time based on the counting results and a processing delay causedby each retransmit.

In addition, the communication control unit 25 may calculate the delaytime due to communication processing of the synchronization messagebased on, for example, information such as on the modulation method orcode rate of the synchronization message.

As a result, the communication control unit 25 may be able to calculatethe delay time caused by communication processing performed in the lowerlayer.

Here, both the retransmit status such as the number of retransmits andthe delay time caused by retransmitting as well as the modulation methodand code rate of data may be simply referred to as the reception status.

The communication control unit 25 reports the reception status of thesynchronization message to the supplemental information generating unit29 and instructs the supplemental information gathering unit 29 togenerate the supplemental message.

If the modulation method and the code rate may be controlled in anadaptive manner depending on the propagation path environment in thecommunication path between the master 2A and the slave 3A, thecommunication control unit 25 may appropriately set the modulationmethod and the code rate of the transmitter 26 and may transmit varioustypes of information depending on the reception quality and so onmeasured by the receiver 27.

That is, the communication control unit 25 functions as an example of acommunication processing unit that controls the communication processingof messages in the transmitter 26 and the receiver 27 in thecommunication layer, which is a lower layer, and outputs informationregarding a delay in the communication processing.

The transmitter 26 transmits various types of information to the slave3A according to an instruction by the communication control unit 25.

The receiver 27 receives the delay information request messagetransmitted from the slave 3A or receives, from the slave 3A, an ACK orNACK indicating whether the slave 3A has successfully received varioustypes of information transmitted from the master 2A.

For example, when the master 2A and the slave 3A are coupled wirelessly,the transmitter 26 and the receiver 27 may be configured as individualor common antennas. In addition, for example, when the master 2A and theslave 3A are coupled through a wire, the transmitter 26 and the receiver27 may be configured as connectors or wired interface devicesappropriate for the transmission path.

The delay information response generating unit 24 obtains, from the timecounter 22, the time when the delay information request message isreceived from the slave 3A, as the time information t4.

At this time, for example, the supplemental information generating unit29 obtains information (reception status information S3) about thereception status of the synchronization message from the communicationcontrol unit 25.

Then, the supplemental information generating unit 29 adds, to the timeinformation t1 included in the supplemental message, delays related toretransmitting and communication processing of the modulation method orthe code rate or so on, using the time information t1 and the receptionstatus information S3 of the synchronization message. More specifically,for example, the supplemental information generating unit 29 obtains thecorrected time t1′ (t1<t1′) by adding the delay related to communicationprocessing with the following expression (10) to the time t1, generatesa supplemental message that includes the corrected time t1′, andtransmits it to the communication control unit 25.t1′=t1+D3  (10)

In the above expression (10), D3 is a delay time that is determined inaccordance with the reception status information S3. For example, whenthe reception status information S3 is the number of retransmits, thesupplemental information generating unit 29 may use for D3 in the abovecorrection a delay time (D3 _(rx)), caused by retransmitting, that iscalculated based on the number of retransmits. When the reception statusinformation S3 is information regarding the modulation method or coderate, the supplemental information generating unit 29 may use for D3 inthe above correction a delay time (D3 _(mcs)), calculated based on themodulation method or code rate, due to communication processing of thesynchronization message.

For example, if 100 bytes of information is transmitted in atransmission path with a transmission speed of 100 kbps when themodulation method is QPSK and the code rate is ½, then D3 _(mcs) isrepresented by the following expression (11).D3_(mcs)=100 Bytes*8/100 kbps=8 msec  (11)

If 100 bytes of information is transmitted in a transmission path with atransmission speed of 100 kbps when the modulation method is 16QAM andthe code rate is ½, then D3 _(mcs) is represented by the followingexpression (12)D3_(mcs)=100 Bytes*8/100*2 kbps=4 msec  (12)

That is, the synchronization information generating unit 30, thesupplemental information generating unit 29, and the delay informationresponse generating unit 24 function as an example of a timesynchronization unit that performs time synchronization with the slave3A based on information regarding delays output from the communicationcontrol unit 25 in the time synchronization layer above thecommunication layer.

In the above example, the supplemental information generating unit 29changes the transmitting time t1 of the synchronization message to t1′.However, for example, the reception status information S3 and thereceiving time information t1 may be transmitted to the slave 3A withthe information put in the supplemental message and then the slave 3Amay correct the time information t1 to t1′ based on the reception statusinformation S3.

An example of operation of the master 2A is described in FIG. 10. Asillustrated in FIG. 10, the supplemental information generating unit 29obtains the transmitting time t1 of the synchronization message from thetime counter 22 (step S31).

Next, the supplemental information generating unit 29 obtains thereception status information S3 from the communication control unit 25and calculates a delay time D3 based on the obtained reception statusinformation S3 (step S32).

Then, the supplemental information generating unit 29 corrects the timet1 of the synchronization message to t1′ based on the calculated delaytime D3 (step S33).

Next, the supplemental information generating unit 29 generates asupplemental message that includes the corrected time t1′ and transmitsit to the slave 3A (step S34).

As described above, the master 2A in this example makes a correction byadding the delay time due to communication processing of thesynchronization message to be transmitted to the slave 3A from the firsttransmitting time of the synchronization message, so that thesupplemental message that includes the corrected time may be transmittedto the slave 3A.

(2.2) Example of the Structure of the Slave 3A

An example of the structure of the slave 3A is illustrated in FIG. 11.The slave 3A in FIG. 11 includes, for example, the slave side clock 31,the time counter 32, the transmitter 36, the receiver 37, and aprocessing unit 38A.

The slave side clock 31 generates a clock signal. The slave side clock31 includes an oscillation circuit having, for example, a quartzoscillator or atomic clock.

The time counter 32 counts the clock signal generated by the slave sideclock 31.

The receiver 37 receives, from the master 2A, a synchronization message,a supplemental message, or a delay information response messagetransmitted from the master 2A, or an ACK or NACK, which indicatewhether the master 2A has successfully received data transmitted fromthe slave 3A.

The transmitter 36 transmits, to the master 2A, a delay informationrequest message or an ACK or NACK, which indicate whether datatransmitted from the master 2A has been successfully received, accordingto an instruction by a communication control unit 35.

For example, when the master 2A and the slave 3A are coupled wirelessly,the transmitter 36 and the receiver 37 may be configured as individualor common antennas. For example, when the master 2A and the slave 3A arecoupled through a wire, the transmitter 36 and the receiver 37 may beconfigured as connectors or wired interface devices appropriate for thetransmission path.

The processing unit 38A performs time synchronization using thesynchronization message, supplemental message, delay information requestmessage, and the delay information response message. Accordingly, theprocessing unit 38A, for example, includes an information analysis unit33A, the delay information request generating unit 34, and thecommunication control unit 35.

The communication control unit 35 transmits the delay informationrequest message generated by the delay information request generatingunit 34 to the transmitter 36 so that this message is transmitted to themaster 2A.

The communication control unit 35 performs retransmit control if thedelay information request message that has been transmitted to themaster 2A is not successfully received by the master 2A. This retransmitcontrol is performed based on an ACK or NACK that the receiver 37receives from the master 2A.

When the synchronization message, supplemental message, or delayinformation response message that has been transmitted from the master2A is successfully received by the receiver 37, an ACK may betransmitted to the master 2A. On the other hand, when thesynchronization message, supplemental message, or delay informationresponse message that has been transmitted from the master 2A is notsuccessfully received by the receiver 37, a NACK may be transmitted tothe master 2A.

In addition, the communication control unit 35 may calculate, forexample, the time from when the delay information request message isfirst transmitted to when the delay information request messagesuccessfully received by the master 2A is transmitted.

Alternatively, the communication control unit 35 may count the number oftimes the delay information request message is retransmitted and thenumber of times a NACK is received from the master 2A, and may calculatethe above delay time based on the counting results and a processingdelay caused for each retransmit.

In addition, the communication control unit 35 may calculate the delaytime due to communication processing of the delay information requestmessage based on information such as on the modulation method or coderate of the delay information request message.

This enables the communication control unit 35 to calculate the delaytime caused by communication processing performed in the lower layer.

In addition, the communication control unit 35 reports both thecalculated delay time due to communication processing of the delayinformation request message and the reception status of the delayinformation request message to the information analysis unit 33A.

If the modulation method and the code rate may be controlled in anadaptive manner depending on the propagation path environment in thecommunication path between the master 2A and the slave 3A, thecommunication control unit 35 may properly set the modulation method andthe code rate of the transmitter 36 and may transmit various types ofinformation depending on the reception quality and so on measured by thereceiver 37.

That is, the communication control unit 35 functions as an example of acommunication processing unit that controls the communication processingof messages in the transmitter 36 and the receiver 37 in thecommunication layer as a lower layer, and also outputs information abouta delay in the communication processing.

The delay information request generating unit 34 generates a delayinformation request message according to an instruction by theinformation analysis unit 33A and transfers the generated message to thecommunication control unit 35 so that the message is transmitted to themaster 2A. The delay information request generating unit 34A alsoobtains the time t3 when the delay information request message isgenerated from the time counter 32 and reports the obtained time t3 tothe information analysis unit 33A.

The information analysis unit 33A obtains, from the communicationcontrol unit 35, the time information t1′ that is included in thesupplemental message transmitted from the master 2A and reception statusinformation S4 that is obtained when the time information t1′ isreceived, and obtains the time t2 (t2<t3) that is the time when the timeinformation t1′ is received, from the time counter 32.

When receiving the supplemental message from the master 2A, theinformation analysis unit 33A instructs the delay information requestgenerating unit 34 to transmit the delay information request message tothe master 2A and receives a notification of the transmitting time t3 ofthe delay information request message from the time counter 32.

When a delay information response message is received from the master2A, the information analysis unit 33A obtains, from the delayinformation response message, the time t4 when the delay informationrequest message was received by the master 2A.

Then, the information analysis unit 33A adds, to the time t3 when thedelay information request message has been first transmitted, delaysrelated to retransmitting and communication processing of the modulationmethod or the code rate or so on using, for example, the timeinformation t3 and the reception status information S4 of the delayinformation request message. More specifically, for example, theinformation analysis unit 33A obtains the corrected time t3′ (t3<t3′) byadding delay related to communication processing to the time t3 usingthe following expression (13).t3′=t3+D4  (13)

In the above expression (13), D4 is a delay time that is determined inaccordance with the reception status information S4. For example, whenthe reception status information S4 is the number of retransmits, theinformation analysis unit 33A may use for D4 in the above correction adelay time (D4 _(rx)), caused by retransmitting, that is calculatedbased on the number of retransmits. When the reception statusinformation S4 is information about the modulation method or code rate,the information analysis unit 33A may use for D4 in the above correctiona delay time (D4 _(mcs)), calculated based on the modulation method orcode rate, that is due to communication processing of the delayinformation request message.

For example, if 100 bytes of information is transmitted in atransmission path with a transmission speed of 100 kbps when themodulation method is QPSK and the code rate is ½, then D4 _(mcs) isrepresented by the following expression (14).D4_(mcs)=100 Bytes*8/100 kbps=8 msec  (14)

If 100 bytes of information is transmitted in a transmission path with atransmission speed of 100 kbps when the modulation method is 16QAM andthe code rate is ½, then D4 _(mcs) is represented by the followingexpression (15)D4_(mcs)=100 Bytes*8/100*2 kbps=4 msec  (15)

When receiving a delay information response message (t4) from the master2A, the information analysis unit 33A calculates the difference (offset)between the time of the master 2A and the time of the slave 3A using thefollowing expression (16) and corrects the time counter 32 using thecalculated offset.Offset={(t2−t1′)−(t4−t3′)}/2={(t2−t1−D3)−(t4−t3−D4)}/2  (16)

The information analysis unit 33A and the delay information requestgenerating unit 34 function as an example of the time synchronizationunit that performs time synchronization with the master 2A based oninformation regarding delays output from the communication control unit35 in the time synchronization layer that is above the communicationlayer.

An example of operation of the slave 3A is described in FIG. 12. Asillustrated in FIG. 12, first, the receiver 37, the communicationcontrol unit 35, and the information analysis unit 33A obtain the timeinformation t1′ that is included in the supplemental message transmittedfrom the master 2A (step S41).

Next, the information analysis unit 33A obtains, from the time counter32, the receiving time t2 of the synchronization message (step S42).

Then, the information analysis unit 33A and the delay informationrequest generating unit 34 generate a delay information request message,and instruct the communication control unit 35 and the transmitter 36 totransmit the delay information request message (step S43).

In addition, the information analysis unit 33A obtains, from the timecounter 32, the transmitting time t3 of the delay information requestmessage (step S44).

In addition, the information analysis unit 33A obtains the receptionstatus information S4 from the communication control unit 35 andcalculates the delay time D4 based on the obtained reception statusinformation S4 (step S45).

Then, the information analysis unit 33A corrects the transmitting timet3 of the delay information request message to the time t3′ based on thecalculated delay time D4 (step S46).

In addition, the receiver 37, the communication control unit 35, and theinformation analysis unit 33A receive the delay information responsemessage from the master 2A and obtain the time t4 (step S47).

Then, the information analysis unit 33A calculates the offset based onthe time t1′, the time t2, the time t3′, and the time t4 (step S48).

As described above, the slave 3A in this example makes a correction thatsubtracts the delay time due to communication processing of the delayinformation request message to be transmitted to the master 2A from thefirst transmitting time of the message, so that the offset may becalculated based on the corrected time.

Since the slave 3A may synchronize with the time of the master 2A bycorrecting the time of the slave 3A using the offset calculated above,the slave 3A may perform time synchronization at higher precision.

An example of operation of the above communication system 1 isillustrated in FIG. 13. As illustrated in FIG. 13, information regardinga delay in communication processing of the synchronization message isreported from a lower layer of the master 2A to an upper layer of themaster 2A and, in this upper layer, a correction from the time t1 to t1′is made. Then, a supplemental message that includes the corrected timet1′ is transmitted from the master 2A to the slave 3A.

In addition, information regarding a delay in communication processingof the delay information request message is reported from a lower layerof the slave 3A to an upper layer of the slave 3A and, in the upperlayer of the slave 3A, a correction from the time t3 to t3′ is made.

In the example described above, the transmitting side of messagescorrects the transmitting times of the messages based on a delay causedby communication processing of the messages, so that the precision oftime synchronization may be improved.

In the communication system 1, the above correction of time informationby the receiving side and the above correction of time information bythe transmitting side may be combined.

[3] Example of the Hardware Structure

FIG. 14 illustrates an example of the hardware structure of the masters2 and 2A.

An interface (IF) unit 201 is an interface unit for communicating withthe slaves 3 and 3A, and includes a wireless or wired interfaces. Aclock 202 is a unit for counting time and includes devices such as aquartz oscillator or atomic clock. A processor 203 is a data processingunit and includes, for example, a CPU (central processing unit) or a DSP(digital signal processor). A memory 204 is a data storage unit andincludes, for example, a ROM (read-only memory) and a RAM (random accessmemory).

As an example, the correspondence between the structures of the master 2illustrated in FIG. 4 and the master 2A illustrated in FIG. 9 and thestructure of the masters 2 and 2A illustrated in FIG. 14 is describedbelow.

The clock 202 corresponds to, for example, the master side clock 21. TheIF unit 201 corresponds to, for example, the transmitter 26 and thereceiver 27. The processor 203 and the memory 204 correspond to, forexample, the time counter 22, the time information generating unit 23,the delay information response generating unit 24, the communicationcontrol unit 25, the synchronization information generating unit 30, thesupplemental information generating unit 29, and the processing units 28and 28A.

FIG. 15 illustrates an example of the hardware structure of the slaves 3and 3A.

An IF unit 301 is an interface device for communicating with the masters2 and 2A and includes a wireless or wired interfaces. A clock 302 is aunit for counting time and includes a device such as a quartz oscillatoror atomic clock. A processor 303 is a data processing device andincludes, for example, a CPU or DSP. A memory 304 is a data storagedevice and includes, for example, a ROM or a RAM.

As an example, the correspondence between structure of the slave 3 inFIG. 6 and the slave 3A in FIG. 11 and the structure in FIG. 15 isdescribed below.

The clock 302 corresponds to, for example, the slave side clock 31. TheIF unit 301 corresponds to, for example, the transmitter 36 and thereceiver 37. The processor 303 and the memory 304 correspond to, forexample, the time counter 32, the information analysis units 33 and 33A,delay information request generating unit 34, the communication controlunit 35, and the processing units 38 and 38A.

[4] Others

The above structures and functions of the masters 2 and 2A and theslaves 3 and 3A may be selectively adopted or may be combined asappropriate. Accordingly, the above structures and functions may beselectively adopted or combined as appropriate so that the abovefunctions according to the present disclosure may be achieved.

For example, the offset between the slaves 3 and 3A may be calculatedaccording to the following expression (17) and the time may be correctedbased on the calculated offset.Offset={(t2′−t1′)−(t4′−t3′)}/2={(t2−D2−t1−D3)−(t4−D1−t3−D4)}/2  (17)

In this case, even when a delay in communication processing occursduring communication processing of any messages, the slaves 3 and 3A maycorrect their times by subtracting the delay in communicationprocessing, thereby making it possible to certainly improve theprecision of time synchronization.

The time synchronization according to this example is performed betweena communication apparatus as a master and another communicationapparatus as a slave in the above example, but time synchronizationaccording to this example may also be performed between, for example, aslave that is in time synchronization with a master, and another slave.

In this case, a slave that is in time synchronization with a master hasthe same structure and function as the master in the above example andthe other slave has the same structure and function as the slave in theabove example.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A communication apparatus comprising: atransmitter that transmits a first message to another communicationapparatus; a receiver that receives a second message from the othercommunication apparatus; and a processor that performs timesynchronization using the first message and the second message; whereinthe processor generates information regarding a delay in a communicationprocessing of the first message in the transmitter and the secondmessage in the receiver in a communication layer in communication withthe other communication apparatus, and the processor performs the timesynchronization with the other communication apparatus based on theinformation regarding the delay in a time synchronization layer abovethe communication layer, wherein the time synchronization includes atleast one of subtracting the delay in the communication processing froma receiving time of the second message received by the receiver andadding the delay in the communication processing to a transmitting timein the first message transmitted by the transmitter.
 2. Thecommunication apparatus according to claim 1, wherein the informationregarding the delay includes information regarding modulation methodsand code rates of the first message and the second message.
 3. Thecommunication apparatus according to claim 1, wherein the informationregarding the delay includes a delay time in retransmitting of the firstmessage and the second message or the number of times that the firstmessage and the second message are retransmitted.
 4. The communicationapparatus according to claim 1, wherein the first message and the secondmessage include one of a synchronization message and a delay informationrequest message.
 5. The communication apparatus according to claim 1,wherein the transmitter transmits a response including a result of oneof the subtracting and the adding to the other communication apparatus.6. The communication apparatus according to claim 1, wherein the timesynchronization uses Precision Time Protocol.
 7. A communication systemcomprising: a communication apparatus including a transmitter thattransmits a first message to another communication apparatus; a receiverthat receives a second message from the other communication apparatus;and a processor that performs time synchronization using the firstmessage and the second message; wherein the processor generatesinformation regarding a delay in acommunication processing of the firstmessage in the transmitter and the second message in the receiver in acommunication layer in communication with the other communicationapparatus, and the processor performs the time synchronization with theother communication apparatus based on the information regarding thedelay in a time synchronization layer above the communication layer,wherein the time synchronization includes at least one of subtractingthe delay in the communication processing from a receiving time of thesecond message received by the receiver and adding the delay in thecommunication processing to a transmitting time in the first messagetransmitted by the transmitter.
 8. The communication system according toclaim 7, wherein the information regarding the delay includesinformation regarding modulation methods and code rates of the firstmessage and the second message.
 9. The communication system according toclaim 7, wherein the information regarding the delay includes a delaytime in retransmitting of the first message and the second message orthe number of times that the first message and the second message areretransmitted.
 10. The communication system according to claim 7,wherein the first message and the second message.
 11. The communicationsystem according to claim 7, wherein the transmitter transmits aresponse including a result of one of the subtracting and the adding tothe other communication apparatus.
 12. The communication systemaccording to claim 7, wherein the time synchronization uses PrecisionTime Protocol.
 13. A communication method comprising: generatinginformation regarding a delay in a communication processing of a firstmessage in a transmitter and a second message in a receiver in acommunication layer in communication with another communicationapparatus, and performing time synchronization with the othercommunication apparatus based on the information regarding the delay ina time synchronization layer above the communication layer, wherein thetime synchronization includes at least one of subtracting the delay inthe communication processing from a receiving time of the second messagereceived by the receiver and adding the delay in the communicationprocessing to a transmitting time in the first message transmitted bythe transmitter.
 14. The communication method according to claim 13,wherein the information regarding the delay includes informationregarding modulation methods and code rates of the first message and thesecond message.
 15. The communication method according to claim 13,wherein the information regarding the delay includes a delay time inretransmitting of the first message and the second message or the numberof times that the first message and the second message areretransmitted.
 16. The communication method according to claim 13,wherein the first message and the second message include one of asynchronization message and a delay information request message.
 17. Thecommunication method according to claim 13, further comprising,transmitting a response including a result of one of the subtracting andthe adding to the other communication apparatus.
 18. The communicationmethod according to claim 13, wherein the time synchronization usesPrecision Time Protocol.